That's what I thought at first, but contextually 戻ってから is a bit confusing. In the story he is planning on pretending to make a round trip to the bathroom, using it as an excuse to be by the door since originally he wanted to leave the house.

But then I'm not sure how "returning" fits.

案としてはやや苦しいが、
The plan was a bit forced, but

大輔は気力の続く限り戻ってから、
to the limits of his spirit/willpower,

往復してごまかすことにした。
he will pretend to be making a round trip (to the bathroom).

I'm not sure how 戻ってから alters the translation, since he's currently going to the bathroom, and isn't returning from a round trip.

The difference is that the sentence doesn’t have “after I return” part.
As the guy says so, this whole thing is written as just a “lame excuse”. Lame excuses (have to) sound redundant, don’t they?

More precisely, the phrase 気力の続く限り only modifies the word "return", not the "round trip". That's the difference. I have no idea what it means because I don't know what happend before that, though.

EDIT:
Let me show you a seemingly similar (possible) situation.

You are cheating on your girlfriend. And you are on a trip to see your new lover.
When your girlfriend calls you and tells you about her suspicion, you decide to “return” to your girlfriend through thick and thin. And, in order to pretend it was originally only a business trip, you are to make a (new) “round trip” or possibly “several round trips” just to persuade her. Makes sense?

Glad to hear you got it.
But you need to return to the train station when you need to return to your office, don’t you? Although it’s kind of hard to tell by only reading this limited paragraph, to me, the staircase doesn’t seem to be his final destination he decided to “return”.

c.f.) “Because I forgot to bring the papers with me from my office, I decided to return. … And then I returned to the station.”
The first sentence does not necessarily mean he decided to go to the station, right?